1. Field of the Invention
This invention relates to an artificial hip joint for artificially remedying the hip joint of a living body and for restoring the function and form of the hip joint
2. Prior Art
When the function of a hip joint is damaged due to an external injury caused by a traffic accident or degeneration of the hip joint bone caused by rheumatism or a tumor, and is difficult to be recovered, the hip joint is generally cut off and replaced with an artificial hip joint.
The artificial hip joint used for the above-mentioned purpose should meet the following requirements: (1) The hip joint must have superior affinity to its surrounding tissue since it is embedded in a living body for an extended period. (2) The hip joint must not be degenerated in the living body, and its characteristics, such as mechanical strength, must remain unchanged. (3) The sliding section of the hip joint must exhibit superior wear resistance during swinging and be capable of performing the basic joint function. Particularly, the section to be embedded in the bone must be strong and the sliding section must have superior impact strength. It has been ascertained that several times as heavy as the living body may be applied to the hip joint when the corresponding muscle is activated and the weight of the living body is applied. Accordingly, the artificial hip joint must have great mechanical strength and its members must be securely connected with the bone of the living body. Many artificial hip joint meeting these requirements have been developed with due regard to combination of materials. More specifically, an alloy steel with high mechanical strength and high corrosion resistance is usually used for the stem of an artificial hip joint, which is embedded in a thighbone. Table 1 shows known combinations of materials for the condyle which is located at the end of the stem and materials for the socket which swingably accommodates the condyle and is embedded and secured in the hipbone.
TABLE 1 ______________________________________ Combination Material example Condyle Socket ______________________________________ No. 1 Metal Metal No. 2 Plastics Metal No. 3 Metal Plastics No. 4 Ceramics Ceramics No. 5 Ceramics Plastics ______________________________________
The artificial hip joint which is made of the materials of combination example No. 1 (metal-metal) has high mechanical strength. However, when the condyle slides (swings) in the socket, metal particles are generated and dissolve into body fluids as metal ions. The metal ions are harmful to the living body and damage the tissue around the hip joint. Therefore, the combination is not suited for use for an extended period. At present, artificial hip joint made of a metal condyle and a metal socket are scarcely used. The artificial hip joint which is made of the materials of combination example No. 4 (ceramics-ceramics) does not cause harm to the living body. However, when an impact load, several times as heavy as the body, is applied to the contact position of the two ceramic elements with a Vickers hardness of 1800, the ceramic elements may frequently break since they cannot dampen impacts and ceramic material is not tough but brittle. This is a grave defect of the artificial hip joint made of ceramic material (combination example No. 4), although its ceramic socket can be directly embedded in and connected to the bone since it is harmless and has superior affinity to the living body.
The artificial hip joint which is made of a plastic socket and a metal or ceramic condyle (combination example No. 3 or No. 5) shows superior impact damping performance since a relatively soft plastic sliding surface contacts a hard metal or ceramic material. Furthermore, particles due to friction are scarcely generated since the socket made of plastic material, such as polyethylene and polypropylene, has superior sliding performance and flexibility. Moreover, the plastic socket is almost harmless to the living body. Therefore, the artificial hip joint (combination example No. 3 or No. 5) is good for practical use. However, the plastic socket has poor affinity to the living body, although it is almost harmless. Accordingly, the socket must be securely attached to the bone using methyl meta-acrylate (bone cement) so that the socket can be embedded and secured in the hipbone. When the bone cement cures, solidification heat of 60.degree.-80.degree. C. generates and damages the surrounding tissue. Even after curing, polymerization proceeds. As time passes, the cement becomes brittle and is prone to disintegrate, thus it loses its effect. As a result, the artificial hip joint may not function as intended, although the hip joint itself is not damaged at all. Furthermore, residual monomer, which is not yet reacted, gradually dissolves and adversely affects metabolism of the bone. Thus the bone contacting the bone cement is absorbed and the connection strength between the socket and the bone decreased as time passes. This fatal defect has not yet been remedied.